1. Field of the Invention
The present invention relates to an ultrasonic tool and an ultrasonic bonder. More specifically, the invention relates to an ultrasonic tool and an ultrasonic bonder having a feature, in its constitution, for improving the durability of the ultrasonic tool attached to an end of an ultrasonic bonder for flip-chip-bonding a semiconductor device or another electronic device, as well as for improving the efficiency for transmitting ultrasonic wave energy.
2. Description of the Related Art
An ultrasonic bonder has heretofore been known for bonding resin materials or metal materials. In an ultrasonic bonder, a base material and a material to be bonded to the base material are overlapped one upon the other, and are bonded together by utilizing the friction and the heat of friction that is generated when the surfaces to be bonded are slid at high frequency utilizing ultrasonic waves (see, for example, JP 2001-105159 A and JP 7-326619 A).
That is, when the junction surfaces are slid at a high frequency, the base material and the material to be bonded melt due to the friction and the heat of friction accompanying the sliding, or the oxide and the like on the surface of the base material and of the material to be bonded are removed, and a firm and strong bond is formed between resin materials or between the metal materials.
The above ultrasonic bonding has heretofore been employed when an electronic part having bumps formed on the lower surface thereof is to be mounted on a substrate. A conventional ultrasonic bonding process will be described below with reference to FIGS. 6A to 6D.
As shown in FIG. 6A, Cu pads 31 connecting to wiring patterns (not shown) are provided on a mounting substrate 30, and a solder resist 32 is provided to surround the Cu pads 31.
As shown in FIG. 6B, an under-filling resin 33 obtained by mixing an epoxy resin and a coupling agent is then applied so as to completely fill the space under an electronic part to be mounted, depending upon the size of the electronic part.
Subsequently, as shown in FIG. 6C, an electronic part 40 provided with Au bumps 42 having Cu pads 41 interposed between the respective bumps and the part is so placed that the Au bumps 42 of the electronic part 40 face the Cu pads 31 of the mounting substrate 30. An ultrasonic tool 51 made of a metal material containing Fe provided at an end of the ultrasonic bonder is then pushed onto the upper surface of the electronic part 40 to press the electronic part 40 to the mounting substrate 30 while applying ultrasonic waves, to thereby bond the Au bumps 42 of the electronic part 40 to the Cu pads 31 of the mounting substrate 30. In the drawing, reference numeral 52 denotes a horn that constitutes the ultrasonic bonder.
As shown in FIG. 6D, the under-filling resin 33 is thermoset to complete the mounting structure.
When the ultrasonic energy is applied while pressing the electronic part as described above, however, only up to about 60% of the applied energy is transmitted to the electronic part when a conventional ultrasonic tool is used. The 60% of energy transmission rate is herein calculated by dividing oscillation of an electronic part by oscillation of an ultrasonic tool.
Referring to FIG. 7, this is presumably due to that a slip occurs at the interface between the ultrasonic tool 51 and the electronic part 40 due to a low frictional resistance at the interface. When the mounting operation is continuously conducted using the above ultrasonic tool 51, both the ultrasonic tool 51 and the electronic part 40 are damaged.
Under such circumstances, the ultrasonic tool had to be removed and regenerated before the development of damage, requiring an increased number of steps and costs.
Besides, due to a large loss in the energy transmission, the bonding quality is not stable.
To enhance the energy transmission rate, in this case, it can be contrived to increase the frictional resistance by roughening the surfaces. If the surfaces are simply roughened, however, the surfaces of the electronic parts are damaged, and the mass-production thereof is hindered.
It is known to provide the surface of an ultrasonic tool with a coating. However, such a coating does not make it possible to improve both the energy transmission rate in ultrasonic bonding and the durability of an ultrasonic tool.